The present invention relates to optical components for shaping the wavefront of a beam of electromagnetic radiation, and to their use in the field of optical communications for information encoding, particularly for optical communications in the visible or near infrared spectrum.
More specifically, the invention relates to the production of geometrical phase optical components and the use of these optical components for generating helical modes of an optical electromagnetic wave.
Optical components for wavefront shaping are normally based on a suitable spatial modulation of the length of the optical path travelled by different portions of the wavefront, such as that provided by passive components such as lenses, curved mirrors and gradient-index (GRIN) components, and also by active components such as spatial light modulators (SLM) of the liquid crystal or other types.
A different and highly versatile approach, in which phase modulation is introduced by diffraction, makes use of holographic components. However, holographic components normally have the drawback of simultaneously generating a plurality of diffraction orders with different wavefronts in addition to the desired wavefront.
There is an approach to the spatial modulation of the optical phase which is completely different from the preceding ones, and which is based on what is known as the “geometric phase” or Pancharatnam-Berry phase.
This relates to a phase delay (additional to that due to the length of the optical path) which an electromagnetic wave acquires when its polarization is subjected to a series of transformations whose initial and final states are identical. This phase is determined solely by the geometry of the closed path travelled in the light polarization space (such as the Poincare sphere). If the polarization of a wave is subjected to transformations which differ from point to point along the transverse profile of the wave, but which start and terminate with a spatially homogeneous polarization state, the wave acquires a clearly defined geometrical phase which differs from one point of its transverse profile to another, and the wavefront is consequently modified.
It has recently been proposed that this effect should be used to produce “geometrical phase” or “Pancharatnam-Berry” optical components (“Pancharatnam-Berry Optical Elements”, PBOE) for modulating the wavefront.
The only PBOE's produced up to the present are based on optical gratings with a pitch smaller than the wavelength (known as “subwavelength gratings”), which limits their application to the mid-infrared domain, using the fabrication methods available today. The specific components which have been produced include beam-splitters, helical mode generators, lenses, all operating at wavelengths in the vicinity of 10 μm.
WO2004/003596 describes such components for the spatial control of the phase of an incident electromagnetic beam as a function of its polarization. The element comprises a substrate with a plurality of grating regions having a pitch smaller than the input wavelength, and having a continuously variable spatial orientation.
However, the visible and near infrared domain is the one which is most useful for applications in the telecommunications field, and it would be desirable to produce PBOE's operating in this wavelength range (approximately from 400 nm to 1700 nm).
A specific type of PBOE which is of particular interest for its potential applications in this optical frequency domain is that of generators of helical modes (such as the Laguerre-Gauss modes).
These particular modes of the electromagnetic field are used at present for the controlled manipulation of micrometric particles in so-called optical tweezers.
The helical state (referred to as “helicity” hereafter for the sake of brevity) of light is considered to be interesting as a possible discrete variable with a plurality of values for encoding information in optical communication in the air or in optical fibres, and for communication protected by quantum cryptography.
The methods conventionally used to generate helical beams in the visible or near infrared domain can be divided into the following three categories:
(i) the cylindrical lens method;
(ii) the spiral plate method;
(iii) holographic methods.
Method (i) is based on the transformation of Hermite-Gauss modes to Laguerre-Gauss modes by passage through two suitably positioned cylindrical lenses. The switching of the helicity state requires the movement of the lenses or switching between different Hermite-Gauss modes, neither of which processes can be automated easily or quickly.
Method (ii) is based on the production of a plate of isotropic glass, machined so as to have a surface in the form of a single helical pitch (like a spiral staircase), which is concluded by a step of suitable thickness, such that it induces a phase delay in the light passing through the plate equal to an exact multiple of 2π. Fine adjustment of the phase delay can be achieved by immersing the plate in a liquid and adjusting the temperature so as to modify the refractive indices of the plate and the liquid. This method does not allow any switching and provides a helical beam with a substantially locked helicity.
Finally, in holographic methods (iii) the beam is diffracted into a hologram suitably designed to produce the desired helical beams in the diffracted orders. It is also possible to obtain different beams simultaneously with different helicities, but travelling in different directions. If it is desired to switch the helicity of a single beam, it is necessary to modify the hologram (which can be done by using holograms generated in a computer-controlled spatial phase modulator, although this does not allow switching rates faster than a few hundreds of hertz), or to modify the direction from which the incident beam arrives. Fast switching can therefore be achieved only by switching the incoming beam between a plurality of beams arriving from different directions, giving rise to obvious problems of complexity and wastage of optical energy. Furthermore, the generation efficiency of holographic methods is no greater than 70%; in other words, they waste at least 30% of the light energy of each incident beam.
Thus all of these methods for generating helical modes in the visible or near infrared domain have the same limitation: they do not allow fast switching of the state of helicity or “order” of the helical mode.
This limitation is particularly serious where possible communications applications are concerned.
The alternative use of PBOE's produced according to the prior art is, as has been said, confined to applications in the mid-infrared domain, and is therefore excluded at present from optical communications applications at the useful wavelengths.
The object of the present invention is to provide a satisfactory solution to the problems described above, i.e. to produce geometrical phase optical elements operating in the visible and near infrared spectral domain.
A further object of the invention is to provide a helical mode generation system, based on geometrical phase optical elements, which allows fast switching of the helicity state of the helical mode, while avoiding the drawbacks of the known art.
According to the present invention, these objects are achieved by means of a geometrical phase optical element having the characteristics claimed in claim 1 and a system having the characteristics claimed in claim 14.